The distribution of ships at sea is a key factor for maritime traffic analysis and ship safety management. With the rapid development of earth observation technology

remote sensing is now considered a useful tool to detect ships at sea on a large scale. Particularly owing to the unique technical properties

such as being less vulnerable to cloud and mist and being unaffected by day and night

synthetic aperture radar (SAR) is widely used for ship detection in maritime security management. However

azimuth ambiguities caused by the mechanism of SAR imaging can be easily misclassified as ships on SAR images

leading to a high false alarm rate in ship detection

which has been a difficult problem in ship monitoring with SAR. Considering this issue

the mechanism of azimuth ambiguities on SAR images was initially analyzed in this study. Then

a new method for azimuth ambiguity removal was proposed based on this mechanism. The removal process of azimuth ambiguities includes three steps. First

the consistency of angles is estimated between the real target and its azimuth ambiguities. In this step

the determination method of azimuth angle between real target and its azimuth ambiguities was also discussed. Second

the uniformity of offset distance is determined

and determining the method of the azimuth distance between the real target and its azimuth ambiguities was also discussed. Third

energy decay is analyzed in the azimuth direction

considering that azimuth ambiguities of real ships on SAR images will follow the principles of energy decay. Using these three discriminant criteria

bright targets detected from SAR images can be classified as real ships and azimuth ambiguities. Radarsat-2 images covering the Bohai Sea or the North of the Yellow Sea were selected for a case study; the spatial resolution of these test images captured from March to June 2015 was 30 m. Using the method proposed in this research

azimuth ambiguities of ships were removed step by step and stored in a geodatabase. Real ship targets were further extracted and stored in a geodatabase. These results were compared with the Automatic Identification System data

which can be considered factual data for the case study. Experimental results indicate that all azimuth ambiguities in the study area were detected and removed from real ship targets. After being tested with four Radarsat-2 images

the average accuracy of this azimuth ambiguity removal method based on spaceborne SAR images proposed in this research is more than 95.8%. The results showed that this method can be effectively used to distinguish real ships from its azimuth ambiguities for 30 m spatial resolution SAR images and can improve the accuracy of ship detection on SAR images.